The present invention relates to growing metal-metal epitaxy on substrates at room temperature. Specifically, the invention relates to metal film and metal superlattice structures with controlled orientations grown at room temperature using a silicon or germanium substrate coated with an epitaxially grown copper layer.
Recent developments of epitaxial growth of metal layers at room temperatures provides a way of obtaining various metal structures for both scientific study and technological applications. The lack of suitable metallic substrates in thin film form has limited progress in the area.
The present invention describes a method which allows growth of metal films and metal-metal alternate structures, or superlattices, with controlled orientations. The method commences with a silicon or germanium substrate coated with an epitaxially grown copper film as the seed layer. The deposition is performed by conventional electron beam evaporation in a vacuum of low 10.sup.-7 torr, without intentional or externally applied heating of the substrate during the deposition process. When a silicon substrate is used, the technique just described is referred to as metal-metal epitaxy on silicon (MMES).
The MMES technique described uses silicon as the substrate with epitaxial grown copper film as the seeding layer for the epitaxial growth of other metals. This method differs from the technique of depositing epitaxial metal films on GaAs substrates by means of molecular beam epitaxy (MBE) technique. The latter process is more complicated and costly than the MMES method. The cost involved of the MBE system and GaAs substrates as well as the growth process involved make the MMES method more attractive and practical for large scale applications. The principles of the MMES method is extendible to other growth techniques, including sputtering and other thin film deposition techniques.
It is known in the art that metal films of controlled orientation are difficult to grow on each other due to the difficulty of obtaining seed metal layers with the desired orientation. According to the present invention, a novel method is disclosed in which a seed metal layer of controlled orientation, for example Cu, is first deposited on single crystal silicon. The subsequent deposition of other metals on a (100) copper film allows the growth of (100) oriented metal films. Metals such as nickel, cobalt, titanium, palladium, rhodium, iridium, zirconium and hafnium have been so deposited. Certain metals do not exhibit a lattice match with the copper seed layer. Such metals require one or more additional metal seed layers before epitaxial growth is achieved. For example, gold, silver, platinum, iron, vanadium and chromium which do not grow epitaxially on copper will grow epitaxially when palladium is first deposited on the copper film. Tungsten and molybdenum, for instance, do not grow epitaxially on palladium but will grow epitaxially when gold is first deposited on the palladium film. The result is a five layer metal-metal epitaxy on silicon structure. Niobium, which shows no epitaxial growth on palladium or gold, shows a partial epitaxy of (100) growth on (100) Mo. This is an example of six layer epitaxy on silicon. The method is performed using conventional electron beam evaporation in a vacuum of 10.sup.-7 torr, without intentional heating of the substrate during deposition. As a result, the method is readily applicable to a manufacturing line.
Following the described method, copper epitaxially deposits on silicon with (100) and (111) Cu films grown using (100) and (111) Si substrates, respectively. Using the deposited copper layer as a seeding layer, further epitaxial growth of other metals is accomplished. Metals which exhibit such an epitaxial relation on the copper layer include Ni, Co, Rh, Ir, Ti, Pd, Zr and Hf. Other metals such as Au, Ag, Pt, Fe, V and Cr, which do not grow epitaxially on copper, grow epitaxially on Pd using Pd/Cu/Si structures. Still other metals such as Mo and W, which do not grow epitaxially on Pd, grow epitaxially on Au using Au/Pd/Cu/Si structures. Still further metals such as Nb, which do not grow epitaxially on palladium or gold, grow epitaxially on Mo using Mo/Au/Pd/Cu/Si structures.
The present invention results in a single deposition of metal films with controlled orientation using conventional thin film evaporation techniques in a vacuum chamber without external heat applied to the substrate during deposition thereby minimizing reaction among components. The metal films thus deposited have many applications.
The magnetic metals such as Ni, Co and Fe, all as thin individual layers and as superlattices, are of interest to the magnetic community. Studies involving Cu-Ni, Cu-Co and Ni-Co superlattices exhibit magnetic properties which are clearly dependent upon orientation. Rh and Ir layers and superlattices have application in catalytic activity of the metal using single crystals of different orientations. Metals such as Ni, Pd and Fe are also of interest as catalysts. The MMES method provides a simple method for making metal films with controlled orientation at much lower cost and with less complication in sample surface preparation than heretofore known methods.
According to the present invention, a seed metal layer of controlled orientation, for example Cu, is first deposited on single crystal germanium. The subsequent deposition of other metals, such as Ni, on a (100) oriented copper film allows the growth of (100) oriented metal film. The results with germanium substrate have been similar to those described above in conjunction with silicon substrates.
The use of germanium as the substrate offers the advantage that it is possible to grow metal-metal structure with controlled orientations on GaAs substrate. In an article entitled "Interface morphology of epitaxial growth of Ge on GaAs and GaAs on Ge by molecular beam epitaxy" by Chin-An Chang in the Journal of Applied Physics, volume 53, page 1253 dated February 1982 and in an article entitled "Interface morphology studies of (110) and (111e Ge-GaAs grown by molecular beam epitaxy" by Chin-An Chang in Applied Physics Letters, volume 40, page 1037, dated June 1982 epitaxially grown germanium on GaAs using molecular beam epitaxy techniques is described. Such a germanium layer can be used as the substrate for the epitaxial growth of Cu, followed by other metals. After the Ge layer has been deposited on GaAs, the metallic layers or superlattices can be grown on the Ge/GaAs substrate either in the same molecular beam epitaxy system or in a separate chamber. The result is the simultaneous presence of both the magnetic metal structures and GaAs device structures on the same substrate for a variety of applications. Alternatively, it is possible to deposit Si on the GaAs substrate, followed by the deposition of the metal structures. In this regard, Ge is preferable due to the excellent lattice metal with GaAs; the mismatch is less than 0.1% for Ge-GaAs but more than 4% for Si-GaAs.